Research on a GaN HEMT On-Board Charger for Electric Vehicles
Thesis for the degree of [Doctor of Philosophy Guoen Cao Hanyang University Graduate School
Research on a GaN HEMT On-Board Charger for Electric Vehicles Guoen Cao Dept. of Electronic Systems Engineering The Graduate School Hanyang University With an accelerating global energy crisis and deteriorating environmental problems, electric vehicle (EV) technologies have attracted growing interest due to their reduced fuel usage and greenhouse emissions. The battery charger plays a critical role for the acceptance and development of EVs. Because a battery is generally used as the main power source, a high conversion efficiency, high power density, and lightweight on-board-charger (OBC) is needed in order to maximize the energy utilization. Gallium nitride based high electron-mobility transistors (GaN HEMTs) are potential candidates as next-generation power switching devices due to the enormous potential use in the applications of high frequency, high temperature, and high output power, in particular of battery charger applications. Although much progress has been achieved in the development of GaN HEMTs, a few important issues such as current collapse effects should be evaluated before wide deployment is possible. Since evaluating performance in power semiconductors and selecting the optimal topologies are important steps in the design and development of power electronics circuits, this thesis is concerned with the performance evaluation of the new GaN HEMTs and the design of an isolated OBC that uses GaN HEMTs to achieve high efficiency for future applications in EVs. GaN HEMTs suffer from current collapse effect in operation regions, which leads the dynamic on-resistance to increase when a high voltage is applied to the transistor. In order to measure the dynamic on-resistance and evaluate the current collapse effect of newdesigned GaN HEMTs, a novel soft-switching measurement circuit based on synchronous buck topology is introduced. To apply high-voltage and high-current stresses to the device without additional spikes and oscillation, the resonance technique has been employed. As a result, the proposed circuit can produce sufficient high frequency switching voltage and current stresses equal to or greater than that would be found in real power applications to the devices with general equipment. In order to achieve accurate measurement of onresistance under high frequency switching operations and eliminate the saturation of conventional methods, an active voltage clamp circuit is developed. A prototype circuit has been built. Experiments conducted under extreme conditions have been carried out. The simulation and experimental results confirm the validity of the proposed circuit. After evaluating characteristics of the new-designed GaN HEMTs, an isolated high efficiency on-board battery charger using these new power devices is presented. The OBC has a two-stage structure, where the first stage is an interleaved boost AC-DC power factor correction (PFC) converter and the second stage is a full-bridge LLC resonant converter. As the GaN HEMT has very low gate threshold voltage, a high-speed isolated gate driver circuit with negative voltage has been developed for the efficient operation of the two stages For the GaN-based PFC converter, circuit modeling and the power stage design method are discussed in detail. To keep high power factors and high dynamic performance under a wide input and output range, a fuzzy logic PI current controller and a non-linear voltage controller based on the circuit model are proposed. A 1.5 kW hardware prototype is developed and a maximum system efficiency of 97.5% is measured while operating with the switching frequency of 200 kHz. The results also show a considerable increase in system efficiency and superior performance of the proposed converter compared to the conventional control methods.
For the full-bridge LLC resonant converter, a novel design method for lithium battery charger applications is proposed. According to the charging characteristics, three operating points are selected for the optimum design. A thorough analysis of design procedure is performed, considering the performance evaluation of GaN HEMTs. A 1.5 kW prototype circuit was built, with an output voltage range of 250 V to 420 V. Experimental results show that high efficiency of 95.9% is achieved by using GaN HEMTs and it has resulted in 0.9% improvement compared to the conventional silicon-based converter. In order to implement digital control and control the two-stage in an effective way, a two-core floatingpoint DSP is employed for the OBC. Keywords Electric vehicles, battery charger, GaN HEMT, evaluation circuit, interleaved PFC, fuzzy logic control, soft-switching, LLC resonant converter